Add suggestion for bad block fragment error

compiler/rustc_parse/messages.ftl

@@ -353,6 +353,7 @@ parse_int_literal_too_large = integer literal is too large
 
 parse_invalid_block_macro_segment = cannot use a `block` macro fragment here
     .label = the `block` fragment is within this context
+    .suggestion = wrap this in another block
 
 parse_invalid_char_in_escape = {parse_invalid_char_in_escape_msg}: `{$ch}`
     .label = {parse_invalid_char_in_escape_msg}

compiler/rustc_parse/src/errors.rs

@@ -333,6 +333,17 @@ pub(crate) struct InvalidBlockMacroSegment {
     pub span: Span,
     #[label]
     pub context: Span,
+    #[subdiagnostic]
+    pub wrap: WrapInExplicitBlock,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
+pub(crate) struct WrapInExplicitBlock {
+    #[suggestion_part(code = "{{ ")]
+    pub lo: Span,
+    #[suggestion_part(code = " }}")]
+    pub hi: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/expr.rs

@@ -2186,6 +2186,10 @@ impl<'a> Parser<'a> {
             self.sess.emit_err(errors::InvalidBlockMacroSegment {
                 span: self.token.span,
                 context: lo.to(self.token.span),
+                wrap: errors::WrapInExplicitBlock {
+                    lo: self.token.span.shrink_to_lo(),
+                    hi: self.token.span.shrink_to_hi(),
+                },
             });
         }